A known optical navigation technique involves illuminating a surface, capturing successive images of the illuminated surface, and correlating the successive images with each other to determine relative lateral displacement between the images. Examples of the optical navigation technique are described in U.S. Pat. No. 5,644,139, entitled NAVIGATION TECHNIQUE FOR DETECTING MOVEMENT OF NAVIGATION SENSORS RELATIVE TO AN OBJECT, and U.S. Pat. No. 6,222,174, entitled METHOD OF CORRELATING IMMEDIATELY ACQUIRED AND PREVIOUSLY STORED FEATURE INFORMATION FOR MOTION SENSING, both of which are incorporated by reference herein. Another optical navigation technique utilizes spatial filtering as described in U.S. Pat. No. 5,729,009, entitled METHOD FOR GENERATING QUASI-SINUSOIDAL SIGNALS. These optical navigation techniques are used to track the lateral movement of a navigation device such as a computer mouse relative to a navigation surface.
In many optical navigation applications there is a need to determine the separation distance between the optical navigation device and the navigation surface. For example, it is desirable to know when a computer mouse has been lifted off of the surface so that the navigation function can be suspended. Suspending the navigation function while the computer mouse is lifted off of the surface enables a user to move a cursor over long distances by “skating” the mouse. With a computer mouse that uses a rolling ball to track lateral motion, there is no need to detect lift off from the surface because the ball stops rolling as soon as it looses contact with the navigation surface. In contrast, a computer mouse that uses optical navigation may continue to track changes in lateral position while the mouse is lifted off of the surface and moved across the surface. Continuing to track changes in lateral position while the mouse is lifted off the surface makes it difficult to skate the mouse.